enrEnthalpy Changes

Question 1

Use ideas about enthalpy changes to suggest why some reactions are exothermic

Answer 1

• bond breaking absorbs energy and bond making releases energy
• if more bonds are made in a reaction than broken then the overall enthalpy change will be exothermic

Question 2

A student carries out an experiment to determine the enthalpy change of combustion of
glucose.
In the experiment, 0.831 g of glucose is burned. The energy released is used to heat
100 cm3
of water from 23.7 °C to 41.0 °C.
(i) Calculate the energy released, in kJ, during combustion of 0.831 g glucose.
The specific heat capacity of water = 4.18 J g–1 K–1
Density of water = 1.00 g cm–3

Answer 2

1. Use Q = mcΔt to work out the enthalpy change

2. Convert joule units obtained to whatever the question asks for (usually kJ, divide by 1000)

Question 3

How do you calculate enthalpy change of combustion from energy change (q) ?

Answer 3

enthalpy change (Q) / moles 
ALWAYS use minus sign as combustion is always exothermic

Question 4

Suggest two reasons why standard enthalpy changes of combustion determined experimentally are less exothermic than the calculated theoretical values

Answer 4

• Heat may be released to the surroundings

- Combustion may be incomplete

Question 5

State Le Chatelier’s principle

Answer 5

the position of equilibrium will shift to minimise the effect of any changes made to the conditions

Question 6

Define Enthalpy Change of Combustion

Answer 6

the enthalpy change for the complete combustion of 1 mole of a substance

Question 7

What are standard conditions?

Answer 7

Pressure of 100kPa, temperature of 298K

Question 8

How do you calculate enthalpy change of formation from data for enthalpy of combustion?

Answer 8

1. Write out balanced equation
2. Draw Hess cycle with arrows going from base elements to the reactants/ products
3. Reverse the necesssary arrows
4. Add up the numbers on the alternate pathway

Question 9

Define Enthalpy Change of Formation

Answer 9

Enthalpy change when 1 mole of a compound is formed from it’s constitutent elements at standard conditions of 298K and 101kPa

Question 10

What do you do when given enthalpy of formation data for every species in equation apart from 1, and given the enthalpy of reaction, and are asked to find the enthalpy of formation of that 1 unknown species ?

Answer 10

1. Draw Hess Cycle, circle mole ratio on unknown
2. Add up all the enthalpy of formations on reactant and product side, then follow Hess cycle route to get enthalpy of reaction minus the unknown
3. Do Enthalpy of reaction - calculated enthalpy of reaction to obtain value for unknown
4. Divide that value by the mole ratio of unknown (e.g. 3 AlCl3 means you have to divide by 3)